In recent years, optical disc apparatuses which applies a light spot to an optical disc thereby to record or reproduce information, such as a CD (Compact Disc) or MD (Mini Disc) have been developed. The optical disc has tracks on/from which the information is recorded/reproduced and the optical disc apparatus records/reproduces the information by making the light spot follow the tracks. The tracks are arranged concentrically or helically at intervals of several micrometers (1.6 μm in the case of CD or MD) in its radial direction. The optical disc apparatus has light spot moving means for moving the light spot in a radial direction of the optical disc at high speed and with high precision, to follow these microscopic tracks. As examples of the light spot moving means, a tracking actuator for radially moving an objective lens which focuses a light spot, and a galvanomirror for changing an angle of an incident light to the objective lens are cited. However, only with such light spot moving means, a movement range of the light spot is limited by the size of the tracking actuator or the objective lens or the like and accordingly the movement range is small. Therefore, the optical disc apparatus usually includes traverse moving means for radially moving an optical head itself which internally contains the objective lens. Generally, control for making the light spot follow the tracks by using the light spot moving means is called “tracking control” and control for making the optical head follow the movement of the light spot by using the traverse moving means is called “traverse control”.
In addition, in the conventional traverse control, a difference between the objective lens position and the center of the optical head is generated as an error signal, thereby to perform the control by using this error signal. A method in which a low-band component of a tracking driving signal supplied to the tracking actuator is used as the error signal is widely adopted. The low-band component of the tracking driving signal shows displacement of the objective lens by the tracking control, on the basis of a position where weight and gravity of the objective lens is balanced. When the moving direction of the tracking actuator is horizontal, it is a signal corresponding to a relative position of the objective lens and the center of the optical head, because the displacement of the objective lens by the gravity from the operation center of the tracking actuator in contrast to the moving direction of the tracking actuator is approximately “0” in this case. However, when the position of the apparatus is set up so as to have a vertical moving direction of the tracking actuator, i.e., when the optical disc apparatus is positioned “longitudinally (vertically)”, the objective lens is displaced downwardly due to its weight, which is referred to as “self-weight dislocation”. The position of the objective lens displaced due to the self-weight dislocation is the position where the weight and gravity of the objective lens is balanced, and the traverse control is executed with this position as a center. Therefore, in the conventional optical disc apparatus, when the self-weight dislocation occurs, the movable range of the tracking actuator is narrowed accordingly, whereby the follow-up characteristics of the tracking control are deteriorated.
Japanese Published Patent Application No.Hei.9-223320 discloses an optical disc apparatus which solves such problems. The optical disc apparatus disclosed therein comprises spot position signal generation means for generating a spot position signal which indicates relative displacement of a light receiving element in the optical head and a light spot, and has a structure of using the spot position signal as an error signal of the traverse control. According to this structure, the traverse control of this optical disc apparatus has a point where the spot position signal is zero as a control target and moves the optical head such that the light spot is positioned at the center of the light receiving element. In constituting the optical head, the center of the light receiving element and the operation center of the tracking actuator are previously arranged so as to coincide with each other. Therefore, the objective lens is always moved around the operation center of the tracking actuator, thereby avoiding the deterioration of the follow-up characteristics of the tracking control.
However, so constructed optical disc apparatus has a problem in stability of the operation at the starting of the operation of the traverse control means. FIG. 14 is a waveform chart showing waveforms of a spot position signal and a traverse driving signal in the conventional optical disc apparatus when the self-weight dislocation occurs. In FIG. 14, at an operation start time 1301 of the traverse control means, the objective lens is displaced due to the self-weight dislocation and the spot position signal has a large value A. When the spot position signal is input to the traverse control means in such a state to start the traverse control, a driving signal 1302 applied to a traverse motor has a high amplitude and becomes oscillatory. In the worst case, when the optical head is moved by the traverse motor, the tracking control is taken off. This is because the frequency band of the traverse control is usually limited to several Hertz or less so as not to follow the eccentricity, the traverse motor has large inertia and it is difficult to be moved or stopped, and the like.